a) Field of the Invention PA1 b) Description of the Prior Art
The present invention relates to a focal plane shutter for use photographic camera in which a shutter base plate is made of a synthetic resin material.
Known as base plates for focal plane shutters are ones which are made of metals and others which are made of synthetic resin materials. As a material for the metallic shutter base plates, it is general to select iron or aluminium which is effective for reducing weights of the focal plate shutters. Further, fixed to the shutter base plates are shafts for rotatably supporting levers to be used for driving shutter blade groups and winding springs therearound. These shafts must be fixed perpendicularly to the shutter base plates and have physical strengths sufficient for withstanding movements of the driving levers. It is therefore desirable that these shafts and the shutter base plates are made of metals.
in these days, however, there are provided focal plane shutters in which shutter base plates are made of synthetic resin materials. The shutter base plates made of the synthetic resin materials have been rapidly adopted owing to their merits that they require low material costs and that they can easily be molded into various forms, but these shutter base plates have physical strengths which are lower than those of the shutter base plates made of metals. For this reason, it is inadequate to fix the above-mentioned shafts to shutter base plate which are simply made of the synthetic resin materials. Under these circumstances, certain proposals have been made to make up for the low physical strength of the shutter base plates made of the synthetic resin materials.
U.S. Pat. No. 4,926,202 and No. 5,159,317 have made proposals to form thick-walled portions within regions to which the above-mentioned shafts are to be fixed. In case of a shutter base plate made of a metal, it is sufficient that such a shaft mounting region has thickness of approximately 0.8 mm even when the shutter base plate is to be used in a focal plane shutter of a high-speed type (having a maximum shutter speed, for example, of 1/4 second). In case of a shutter base plate which is made of an ordinary synthetic resin material and manufactured by an ordinary method, however, the shutter base plate must have thickness of at least 1.4 mm, or exceeding 2 mm in certain cases, when it is to be used in a focal plane shutter having a maximum speed of 1/4 second or lower, except for cases where the shutter base plate is made of an exceptionally expensive material and manufactured by an exceptionally expensive method. Such thickness of the shutter base plate made of the synthetic resin material is extremely disadvantageous for configuring a shutter mechanism so as to have a small size in the back-forth direction (a size as measured in a direction along an optical axis of a photographic lens system for a camera).
Further, when the thick-walled portions of a shutter base plate made of a synthetic resin material are thickened, shrink marks are produced due to large variations of internal plasticities during cooling to be performed immediately after molding, thereby making it remarkably difficult to fix the above-mentioned shafts perpendicularly to the shutter base plate. This applies similarly to a shutter base plate in which the above-mentioned shafts are made of a synthetic resin material and formed integrally with the shutter base plate. This defect will be described below with reference to a example illustrated in FIG. 5.
Shown in FIG. 5 are shafts which protrude from a front surface and a rear surface of a shutter base plate 101: a shaft 101a for rotatably supporting a driving lever 102 for a leading blade group, a shaft 101b for rotatably supporting a driving arm 103, a shaft 101c for rotatably supporting a driving lever 105 for a trailing blade group and a shaft 101d for rotatably supporting a driving arm 106. It is required that the shafts 101a and 101c which protrude from the front surface are coaxial with the shafts 101b and 101d respectively which protrude from the rear surface, or the former two must not be eccentric from the latter two.
When the thick-walled portions are formed in the vicinities of roots of the shafts 101a and 101c for obtaining required strengths of these shafts, however, internal plasticities are largely varied (shrink marks are formed) during the cooling performed immediately after the molding, thereby making it is impossible to perpendicularly fix the shafts 101a, 101b, 101c and 101d, making the shaft 101a eccentric from the shaft 101b, making the shaft 101c eccentric from the shaft 101d, and making it impossible to perform correct exposure by operating the lading blade group and the trailing blade group.